As the requirement for greater data transmission capacity over an optical fibre increases, system designers are increasing the bit-rates of transmission systems and also using wavelength division multiplexing (WDM) arrangements so that a number of signals can be transmitted at different wavelengths in a single optical fibre. This requires that optical transmitters are available in each of the transmission wavelengths, both to build a transmission system and for stocking as spare parts. These activities would be simplified if tuneable optical transmitters were available, the output wavelength of which could be controlled simply by the application of a signal such as an electrical voltage. Known tuneable optical devices are difficult to manufacture and hence tend to be expensive.